1. Field of the Invention
The present invention relates to a method for producing a homogeneous polycarbonate composition. More particularly, the present invention is concerned with a method for producing a homogeneous polycarbonate composition of a polycarbonate and an additive, which comprises feeding a main polycarbonate in a molten state to a first inlet of an extruder, while feeding a resin/additive mixture of an auxiliary polycarbonate and at least one additive to a second inlet of the extruder, wherein the second inlet is disposed downstream of the first inlet, and extruding the main polycarbonate and the resin/additive mixture through the extruder. By the method of the present invention, additives can be uniformly dispersed in a molten polycarbonate, so that a polycarbonate composition having various excellent properties can be produced efficiently. For example, when a thermal stabilizer is added to and mixed with a molten polycarbonate by the method of the present invention, a polycarbonate composition having excellent thermal stability can be produced efficiently.
2. Prior Art
In recent years, polycarbonates have been widely used in various fields as engineering plastics which have excellent heat resistance, impact resistance and transparency. Various studies have been made with respect to processes for producing polycarbonates. Up to now, processes, such as one utilizing interfacial condensation polymerization of an aromatic dihydroxy compound, such as 2,2-bis(4-hydroxyphenyl)propane (hereinafter, frequently referred to as "bisphenol A"), with phosgene (hereinafter, frequently referred to as the "phosgene process"), have been commercially practiced. In the phosgene process, a mixed solvent of water or an aqueous alkali solution and a water-immiscible organic solvent are generally used (a mixed solvent of an aqueous sodium hydroxide solution and methylene chloride is practically employed as the mixed solvent); a tertiary amine or a quanternary ammonium compound is employed as a catalyst; and chlorine-containing by-products, such as hydrogen chloride are formed, and for example, hydrogen chloride is removed as a salt thereof with a base.
However, in the phosgene process, (1) toxic phosgene must be used; (2) due to the by-produced chlorine-containing compounds, such as hydrogen chloride and sodium chloride, an apparatus used is likely to be corroded; (3) it is difficult to remove impurities, which adversely influence the properties of the desired polycarbonate, such as sodium chloride, from the polymer; and (4) since methylene chloride (which is generally used as a reaction solvent) is a good solvent for polycarbonate and has a strong affinity to polycarbonate, the methylene chloride inevitably remains in produced polycarbonate. With respect to the remaining methylene chloride, removal thereof is extremely costly, and complete removal of the remaining methylene chloride from the obtained polycarbonate is almost impossible. Further, it is noted that the methylene chloride remaining in the polymer is likely to be decomposed, e.g., by heat at the time of molding the polymer, thereby forming hydrogen chloride, which not only causes corrosion of a molding machine but also lowers the quality of a molded product.
As mentioned above, the phosgene process involves too many problems, so that it has been desired to develop a process for producing a polycarbonate, which is free from difficulties inevitably accompanying the phosgene process.
Recently, a large number of researches and developments have been made with respect to a transesterification process for producing a polycarbonate from an aromatic dihydroxy compound and a carbonic diester, especially a melt transesterification process, which is attracting attention as a substitute for the phosgene process.
It is known that the melt transesterification process is advantageous in that a polycarbonate is obtained in a molten state at the time of completion of the polymerization reaction. For example, a polycarbonate obtained in a molten state can be directly subjected to pelletization and, hence, production of polycarbonate pellets can be conducted efficiently [see "Plastic Zairyo Koza 5, Polycarbonate Jushi (Lecture on Plastic Materials 5, Polycarbonate Resin)", p.62-67, published from The Nikkan Kogyo Shimbun Ltd. (The Daily Industrial News), Japan, 1969]. When it is desired to incorporate an additive into a polycarbonate in pellet form, it is necessary to mix polycarbonate pellets with an additive and melt the resultant mixture so as for the molten mixture to be melt-kneaded in an extruder. This operation is troublesome and economically disadvantageous. Therefore, it has been attempted to produce a polycarbonate/additive composition before a polycarbonate obtained in a molten form by a melt transesterification process is fabricated into pellets.
With respect to the method for producing a polycarbonate/additive composition obtained in a molten form by a melt transesterification process, wherein the additive, for example, a thermal stabilizer is incorporated, there can be mentioned Unexamined Japanese Patent Application Laid-Open Specification (Kokai) No. 5-310906 (corresponding to EP Publication 559953 and U.S. Pat. Nos. 5,278,279, 5,387,628 and 5,391,690) and Examined Japanese Patent Application Publication (Kokoku) No. 5-46843, which disclose a method for adding a phosphorus-containing thermal stabilizer to a polycarbonate obtained in a molten form by a melt transesterification process, wherein the thermal stabilizer is added during the melt polymerization reaction. Further, another method for mixing a polycarbonate obtained by a melt transesterification process with additives is disclosed in Unexamined Japanese Patent Application Laid-Open Specification (Kokai) No. 4-103626, wherein a thermal stabilizer is added to a polycarbonate in a molten state immediately after completion of the melt polymerization reaction.
However, the method disclosed in Japanese Kokai No. 5-310906 and Japanese Kokoku No. 5-46843 has problems in that a lowering of the polymerization rate occurs due to the presence of the thermal stabilizer, and that the polycarbonate composition containing the polycarbonate and the thermal stabilizer suffers discoloration because decomposition and other undesired reactions of the thermal stabilizer occur during the polymerization reaction. On the other hand, the method disclosed in Japanese Kokai No. 4-103626, in which a thermal stabilizer is added after completion of the polymerization reaction, would be free from the above-mentioned problems accompanying the techniques disclosed in Japanese Kokai No. 5-310906 and Japanese Kokoku No. 5-46843. However, this prior art document only describes that a thermal stabilizer and a polycarbonate are kneaded using a twin-screw extruder, but contains no description as to how to add the thermal stabilizer to the polycarbonate and achieve intimate blending of the former with the latter.
On the other hand, with respect to a polycarbonate obtained in the phosgene process (wherein the polycarbonate is obtained in powder or slurry form), it is relatively easy to blend the polycarbonate with an additive, such as a thermal stabilizer, uniformly and obtain a homogeneous composition. Illustratively stated, in the case of a phosgene-process polycarbonate in powder or slurry form, first, a thermal stabilizer which is either dissolved in a solvent, such as methylene chloride, or not dissolved, is added to the polycarbonate; second, the polycarbonate and the thermal stabilizer are then mixed well in a batchwise manner by using, for example, a Henschel mixer, to thereby obtain a mixture; and, third, the obtained mixture is subjected to melt extrusion, thereby obtaining a homogeneous polycarbonate/thermal stabilizer composition. Thus, with respect to a polycarbonate obtained by the phosgene process, it is possible to obtain a homogeneous polycarbonate composition comprising the polycarbonate and an additive (e.g., thermal stabilizer), even when the additive is used in a very small amount. However, as mentioned above, the phosgene process involves too many problems.
As mentioned above, with respect to a polycarbonate obtained in a molten state by a melt transesterification process, it is very difficult to uniformly blend a molten polycarbonate with a small amount of an additive.
Generally, in formulation of a composition of a polycarbonate and an additive, such as a thermal stabilizer, the additive is added in an extremely small amount, specifically in an amount as small as 0.0001 to 0.1 part by weight, relative to 100 parts by weight of the polycarbonate. Therefore, in producing polycarbonate compositions from polycarbonates in molten form and additives, such as a thermal stabilizer, the following various difficulties are encountered.
When a thermal stabilizer in a solid state at room temperature is added to a polycarbonate in a molten state, the thermal stabilizer is heat-melted in a feeder having a pump and fed to an extruder by using the pump, and in the extruder, the thermal stabilizer is mixed with a molten polycarbonate. When a thermal stabilizer is heat-melted in a feeder, the thermal stabilizer is inevitably kept in a molten state for a relatively long period of time, so that the thermal stabilizer deteriorates by heat to suffer discoloration and, hence, the resultant polycarbonate composition, containing such a discolored thermal stabilizer, also suffers from discoloration. On the other hand, when a thermal stabilizer which is in a liquid state at room temperature is added to a polycarbonate, the thermal stabilizer is fed to an extruder by using a feeder having a pump, and in the extruder, the thermal stabilizer is mixed with a molten polycarbonate. In this case, there is a problem such that when the thermal stabilizer is used in a very small amount, the thermal stabilizer cannot be dispersed uniformly, so that when the obtained polycarbonate composition is subjected to molding (in which the composition is heat-melted), the polycarbonate composition suffers discoloration because of its non-homogeneity. On the other hand, a method has been proposed in which a thermal stabilizer which is either in a solid state or in a liquid state at room temperature is dissolved in a solvent, such as methylene chloride, so as to increase its volume, and is fed to a liquid inlet of an extruder. By this method, a thermal stabilizer is well dispersed in a polycarbonate. However, the solvent used for dissolving the thermal stabilizer remains in the obtained polycarbonate composition in an amount as large as several tens to several thousands ppm. A polycarbonate composition containing such a large amount of solvent has poor thermal stability, so that not only is the composition likely to suffer discoloration at the time of molding, but a molded article is also likely to suffer discoloration when it is used under high temperature conditions.
Further, when a thermal stabilizer in powder or granular form, as such, is fed to an auxiliary inlet of an extruder (which auxiliary inlet is provided separately form a main inlet for a polycarbonate and is originally intended for use in feeding such a type of additive as used in a relatively large amount, e.g. a filler, a glass fiber or a carbon fiber), the feeding rate of the thermal stabilizer cannot be controlled precisely at a predetermined level because the amount of the thermal stabilizer to be fed to the extruder per unit time is so small that it is extremely difficult to feed the thermal stabilizer precisely to an extruder by means of a feeder thereof. Therefore, the thermal stabilizer cannot be dispersed uniformly in a polycarbonate, so that the thermal stability of the obtained polycarbonate composition is poor and, hence, not only does the composition suffer discoloration at the time of molding, but a molded article is also likely to suffer discoloration when it is used under high temperature conditions. Further, no feeder is available which can be suitably used for feeding a thermal stabilizer in a very small amount.
As mentioned above, it is very difficult to uniformly disperse a small amount of an additive in a molten polycarbonate obtained by the melt transesterification process, especially when the additive is used in a very small amount. When the additive is dissolved in a solvent and mixed with the polycarbonate, the additive can be well dispersed in the polycarbonate. However, as mentioned above, when a thermal stabilizer is dissolved in a solvent and mixed with the polycarbonate, the solvent remains in the resultant polycarbonate composition, so that the composition has poor thermal stability. Thus, it has conventionally been impossible to uniformly disperse a small amount of an additive in a molten polycarbonate obtained by a transesterification process, so that a polycarbonate composition having various excellent properties, such as excellent thermal stability has not been obtained.